For those of us on Hemodialysis, it is important to know some of the basics involved. Since it is our life at stake here, it serves us well to be familiar with the basic concepts of dialysis. This will help us individualise our treatment and make it work best for us.
There are broadly four areas that we need to be familiar with:
- Dry weight and Ultrafiltration
- Dialysate Sodium
- Blood flow rate and dialysate flow rate
- Dialyzer type and needle gauge
Let us now look over each of these in detail.
1. Dry weight and Ultrafiltration
When your kidneys are not working as well as they should, they don’t remove all the water you drink. Some or all of it stays inside your body. This water is partly in the blood, partly in the body’s tissues and partly in between the tissues.
This has a number of undesirable side-effects – your blood volume goes up, your feet and hands may swell and you could also get breathless because the excess fluid could go and accumulate in your lungs. Not everyone will experience all these symptoms. Each one of us can experience some or all of these symptoms.
Dry weight is your body weight assuming there is not one drop of excess fluid that has not been removed by your kidneys. It is your weight assuming your kidneys were working. Now it is difficult to tell exactly what your dry weight is because your kidneys aren’t working. So, that leaves it to indirect methods to figure out our dry weight!
Since there is extra fluid in your blood, the amount of blood your heart has to pump is not normal. The average human body has around 5 litres of blood. Even if you are putting on about 2 litres between sessions, that’s a whole 40% more volume for the heart to pump. This cannot be good. If you’re putting on about 4-5 litres then it is 100% more. This is much worse.
Now, on dialysis, one of the most important things that happens is the removal of this excess fluid. The process of removing this excess fluid from your body by the dialysis machine is called ultrafiltration.
The body is capable of losing fluid during dialysis only at rates of about 400 ml/hour or less.
So, if you are about 2 kg over your dry weight, you’re removing fluid at 500 ml per hour – assuming you do four hours. If you’re 4 kg over your dry weight, you would be removing fluid at 1 litre per hour. This is very dangerous for your body especially your heart.
So, it is very important for you to be aware of your dry weight, to know how you feel when your dry weight has gone up (because, maybe you’ve been eating well) or down (you’ve been exercising) and have complete control over how much water you are removing every session. Discuss your Ultrafiltration Goal (also called UF) with the person who is setting it and arrive at a number keeping all this in mind.
Remember – remove too much – and chances are that you could get a very low Blood Pressure and cramps; remove too little – and you will have fluid in your body when you get off – which means you cannot drink as much until you get your next session.
2. Dialysate Sodium
The Dialysate Sodium setting in a dialysis machine controls how much Sodium the blood is exposed to during a dialysis session.
The dialyzer is the artificial kidney that does the actual work of cleaning our blood off the excess fluid and toxins. How does this actually happen? There are two compartments in the dialyzer – the blood compartment and the dialysate compartment. The blood flows through the blood compartment which contains hundreds of ‘hollow fibres’ which are very thin pipe like structures which have many, many pores on their walls. All around these hollow fibres is the dialysate compartment through which a special solution flows. The excess fluid and the toxins flow through these pores from the blood compartment to the dialysate compartment.
The figure here shows how substances in solutions move from areas of high concentration to areas of low concentration.
Why doesn’t the blood itself leak out of the pores? That’s because the pores are very, very tiny and the size of the blood cells are larger than the size of the pores. The pores are designed so that only the toxins and water can pass through them.
Now, it is possible that things from the dialysate also pass through these pores and enter the blood. It all depends on the size of the substance and the concentration difference on the two sides of the pores. Any substance can only move from one side to the other if the concentration (the amount of the substance per unit volume) is more on one side than the other. If the concentrations are the same, no movement will happen.
Now coming back to Sodium. The dialysate must be ideally designed such that the concentration of Sodium in it is around the same as that of a healthy human body. This is around 135 to 145 mEq/litre. The dialysate, therefore must also be maintained around this level.
The dialysate is prepared by mixing three liquids inside the machine – the Acid solution (Part A), the Bicarbonate solution (Part B) and RO water in a certain proportion.
The Dialysate Sodium can be set in a HD machine. This controls the concentration of Sodium in the dialysate. As in anything related to dialysis, as indeed, in medicine, every individual is different and there is no one single number for Dialysate Sodium that is suitable to all. Generally, a high Dialysate Sodium causes excess sodium to be present in the dialysate and as a direct consequence, in our blood while a lower Dialysate Sodium cause lower amounts of sodium to be present in the dialysate and in our blood. Excess sodium causes excess thirst and causes us to drink more water while low sodium causes low blood pressure and cramps as well.
So, if you come back with excess fluid weight gain, it is quite likely that your Dialysate Sodium setting was higher than you need it to be.
Generally the Dialysate Sodium is recommended to be in the range 137 to 142 mEq/L. If you are getting cramps or low blood pressure, you generally increase the Dialysate Sodium setting up 1 mEq/L at a time. If, on the other hand, you are feeling too thirsty, try going down a notch at a time and see how it feels. Always remember to discuss any changes you make with your nephrologist since he or she is the most aware of your overall condition. What is explained here is only a general guideline.
Some studies have shown a lower rate of cardiovascular problems with lower Dialysate Sodium compared to higher values.
Remember, it your health on the line. It is your body that is being dialyzed. Take more interest in your dialysis prescription and session parameters. So, read up all you can on this and be proactive about your health.
3. Blood flow rate and dialysate flow rate
Like was mentioned earlier, the dialyzer has two compartments – the blood compartment and the dialyzer compartment. The blood flows through the blood compartment and a special fluid called the dialysate flows through the dialysate compartment. The dialysate is prepared by the dialysis machine by mixing three liquids – the acid solution, the bicarbonate solution and pure water in a certain proportion.
The most important part of our dialysis happens in this dialyzer. This is the artificial kidney which is performing the most important task the kidney performs – that of cleaning the blood of the excess toxins and water.
Since the cleaning happens in the dialyzer, it is important that more and more blood passes through the dialyzer from the body. There is a pump that pumps blood out of the body through the arterial needle. The faster the pump rotates, the greater the quantity of blood that is pumped through the dialyzer and the better the blood is cleaned.
Similarly, since the dialysate is the fluid that is surrounding the blood compartment, the volume of dialysate also impacts the amount of cleaning that happens. More dialysate means more toxins can be removed from the blood. The dialysate flow rate can also be controlled by the machine based on a setting.
If you are getting four hours of dialysis, thrice a week (and that is the bare minimum anyone with CKD on Maintenance hemodialysis must get), the number of hours you are spending on the machine is limited to twelve per week. So, it is important for you to ensure that you get the best dialysis possible during those twelve hours. The blood flow rate and dialysate flow rate are by far the most important parameters that affect the cleaning of your blood of toxins. Ultrafiltration rate is not affected by these parameters at all. But the clearing of toxins is almost entirely dependent on these two parameters.
One thing to be careful about is to make sure you don’t go overboard with these changes since too high a blood flow rate could have some negative consequences as well. Higher blood flow rates can be harmful for your AV Fistula. Make sure you discuss with your nephrologist before making any changes. Make any changes only gradually. The human body is very individualistic. Each one has its own quirks. What I have explained here are only general guidelines.
One thing is for sure though – the higher your blood and dialysate flow rate – the better your clearance.
4. Dialyzer type and needle gauge
Dialyzer type is a very important factor in determining whether you are getting good dialysis. As you know, this is akin to an artificial kidney where the actual filtering takes place. This is where the excess fluid and toxins are removed.
There are many types of dialyzers in the market today. The important thing to check is the surface area of the dialyse and whether your dialyzer is low-flux, medium-flux or high-flux.
The surface area of the dialyzer dictates how much space is available for the blood and the dialysate to interact. The greater the surface area, the better the clearance of toxins.
High-flux dialyzers offer the best clearance among all variants followed by medium-flux and then low-flux. High flux dialyzers are also able to remove more fluid easily compared to low flux dialyzer which have constraints in fluid removal. Though fluid removal is a setting on the machine, this is done by altering the Trans-membrane pressure (TMP) and low flux dialyzers cannot withstand a high TMP.
Clearance of middle molecules is something that everyone on dialysis must be worried about. Middle molecules do not have an immediate impact like potassium or sodium but over a long period of time, they can cause something called Carpal Tunnel Syndrome and other problems. So, it is important to make sure we are getting adequate clearance of middle molecules. The clearance of middle molecules is not at all satisfactory in low flux dialyzers. High flux dialyzers do very well in clearing middle molecules. However, dialysis duration plays a very important role in clearing middle molecules as well.
High flux dialyzers are unfortunately and expectedly, much more expensive than the low and medium flux dialyzers.
The gauge of needles is another important factor to consider while determining your dialysis prescription. Dialysis needles are thick. Thicker needles allow more blood to be removed from your vein than thinner needles, obviously. So, using a needle with a thicker diameter allows you to run your dialysis at a higher blood flow rate. Trying to run at high blood flow rates with a thin needle can cause your blood to hemolyze which is quite dangerous. So, it is very important to choose the right type of needle with the right blood flow rate.
One thing to remember is needle gauges are reverse in order. The higher the gauge, the thinner the needle. 16 gauge is thinner than 15 gauge and 16 gauge is thicker than 17 gauge!
In India, most people use 16 gauge needles because we run at low blood flow rates compared to the US where most people use 15 gauge needles because they run at higher blood flow rates.
You should discuss both these things with your nephrologist and arrive at a considered, well-thought out decision than just accept the default that is used for all.
Remember, medicine is highly individual. What works for one person will not necessarily work for all!